Perchlorate-contaminated groundwater could be a widespread legacy of the U.S.'s agricultural past, according to researchers who have pioneered perchlorate forensics. The researchers, led by John Karl Bhlke of the U.S. Geological Survey, used isotopes and other geochemical tracers to identify perchlorate sources. The impact of the historic use of Chilean nitrate fertilizer from the Atacama Desert, which contains naturally occurring perchlorate, is emerging from studies such as one published recently in ES&T.

The study, which identifies historic use of the fertilizer as the most likely cause of groundwater contamination in some areas of Long Island, New York, is one of the first published reports on the use of such forensics in the field. Similar studies are under way in California, Iowa, Arkansas, and New Jersey, but these are part of ongoing litigation, according to coauthor Neil Sturchio of the University of Illinois Chicago. The Long Island study "is a beautifully conceived and executed work that will be helpful to pinpoint sources in some other cases, as well," says analytical chemist Purnendu ("Sandy") Dasgupta of the University of Texas Arlington.

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Perchlorate can come from many sources, including the manufacture and use of fireworks and road-flare runoff. But two sources-military-industrial sources and Atacama fertilizer-stand out because of the enormous quantities of perchlorate they probably released into the environment in the past, according to Dasgupta. Several years ago, he used historical records to estimate that Chilean nitrate fertilizer likely accounts for more low-level perchlorate contamination in the U.S. than military-industrial sources.

Perchlorate from different natural and synthetic sources has distinctive chlorine and oxygen isotopic signatures, and the signature of Atacama fertilizer can be distinguished from other sources. At current and former agricultural sites, the researchers found deeper groundwater, more than 20 years old, containing perchlorate with Atacama's distinctive isotopic signature.